This invention pertains to an improved method for forming vertical recording media, i.e.--magnetic tapes or discs, using magnetically-assisted sputtering techniques. It also pertains to certain improvements in apparatus adaptable for practice of the improved method.
Because of the economies involved, magnetic data storage continues to be an attractive alternative to solid-state data storage. Magnetic media employed in magnetic data storage are generally classifiable by the manner in which the data is recorded thereupon. The first such class is longitudinal recording and refers to the fact that the magnetic layer is magnetizable only in a direction generally parallel to the surface of the magnetic tape or disc. The other class is referred to as vertical recording and signifies that the magnetic layer is magnetizable in a direction generally perpendicular to the surface of the magnetic tape or disc. Because it is theoretically feasible to achieve data bit densities as high as 3.times.10.sup.8 bits per square inch using vertical recording, as compared with a theoretical limit of 2.5.times.10.sup.7 bits per square inch using conventional longitudinal recording, tremendous impetus exists for the development of vertical recording media.
Vertical recording media derives its unique behavior from a magnetic property known as uniaxial magnetic anisotropy. This property is found in materials having closely packed hexagonal crystal structures and which are magnetizable along a single axis of the crystal. Because cobalt exhibits a substantial degree of uniaxial magnetic anisotropy, it has been used to grow perpendicularly anisotropic magnetic films for vertical recording.
In order to produce vertically magnetic films it is necessary that the anisotropy field be greater than the demagnetization field normal to the surface of the tape or disc. This is usually attained by introducing a non-magnetic metal, such as chromium, into the cobalt hexagonal structure. Chromium is typically introduced in the amount of about 12 to 20 percent.
Hitherto, the formation of vertical recording media utilizing magnetically-assisted sputtering techniques has required substantial modification of conventional sputtering apparatus. Such modifications were necessitated by the fact that a magnetic sputtering target tends to shunt the lines of magnetic flux produced by the devices, thus precluding the realization of the enhanced efficiencies obtainable by magnetically-assisted sputtering.
An example of magnetically-assisted sputtering apparatus for avoiding the aforementioned magnetic flux shunting problem, while at the same time permitting the formation of vertical recording media, is set forth in Kadokura et al., "Deposition of Co-Cr Films for Perpendicular Magnetic Recording by Improved Opposing Targets Sputtering", IEEE Transactions on Magnetics, Vol. Mag-17, No. 6, November 1981, pp 3175-3177. Briefly, therein is described sputtering apparatus including a spaced pair of facing cathodes each having a cobalt-chromium target (18.5% Chromium) attached thereto. Positioned to one side outside the volume defined by the space between the facing targets is a substrate upon which target material is to be deposited. The actual deposition process is otherwise conventional and material ejected from the target is caused to impinge upon the substrate, thus forming vertical recording media.
Another example of apparatus for avoiding the aforementioned magnetic flux shunting problem is set forth in Meckel, U.S. Pat. No. 4,299,678 and Meckel et al., U.S. Pat. No. 4,324,631. Both Meckel and Meckel et al. teach that the magnetic flux shunting problem which arises when using a magnetic target with conventional magnetically-assisted sputtering apparatus can be substantially mitigated by heating the magnetic target to near or above its Curie temperature, thus temporarily rendering the target non-magnetic. While in such a non-magnetic state the target does not shunt any appreciable amount of magnetic flux and, thus, the magnetic flux is able to penetrate and project beyond the volume of the target to thereby densify the sputtering plasma. While Meckel and Meckel et al. do teach that cobalt is a magnetic material which will act to shunt magnetic flux, they each fail to note that the Curie temperature for Cobalt is in the order of about 1100.degree. C. Nothing is contained therein which suggests how to obtain a cobalt-containing magnetic film without having to heat the target to such an extreme temperature.